The C++17 equivalent would be something like the following (not tested):

  using NumberExpr = int;
  using VarExpr = std::string;
  struct AddExpr;

  using Expr = std::variant<NumberExpr, AddExpr, VarExpr>;

  struct AddExpr {
    std::unique_ptr<Expr> a;
    std::unique_ptr<Expr> b;
  }

Of course, this being C++, you need forward declarations and a firm grasp of the rules of incomplete types to be confident about declaring a simple AST type.